Physiology of Growth & Development

CHAPTER: 13

Physiology (Physical Aspect) of Growth & Development

Plant Growth, Development & Differentiation:

Plant Growth:

•      Plant Growth may be defined as an increase in size, morphology and weight.

•      F.W. Went (1962) define growth as an increase in cell number, size, and developmental differentiation.

•      Growth involves an increase in dry mass, duplication of protoplasm, cell multiplication, and a permanent increase in volume.

•      Growth is a process in which the organism passes in orderly stages of development through time and the internals necessary to encompass the various critical stages.

•      Growth , development, and differentiation in the plant is the outcome of highly organized and specialized physiological process which are guided by the growth substances to bring change.

•      Growth is the permanent and irreversible increase in volume and weight about by cell division, cell elongation and increase in protoplasm.

•      Increase of organic matter or formation of new protoplasm is basis to the phenomenon of growth.

Elements of Growth:

1. Growth is outcome of specialized and organized physiological biochemical reactions.
2. Permanent and irreversible increase in volume and weight.
3. Cell division and cell elongation in meristematic zones.
4. It can be measured with the help of auxanometer .
5. Growth zones are limited .(It occurs only in meristematic tissues)

Development:

•      The morphological changes which accompanied the growth in size is called development.

•      “It may be defined as the qualitative process as by which undergo progressively and orderly change in structure and physiology during their entire course of life.”

•      It is a sequential series of changes taking place in the plant.

•      It can be controlled by the factors as genetical hormone and environment.

•      Exact and direct measurement can not be taken.

Differentiation:

•      It is the establishment of localized quantitative differences in biochemical metabolic processes and structural organization in cells, tissues and organs.

•      It is the structural and functional modification of unspecialized cells to specialized cells.

•      These differentiation is brought about by growth regulators.

•      They are reversible.

•      Do not lose the nucleic material and have full genetic potentiality (totipotency)

•      It cannot be measured

Ø  However, the growth , development and differentiation are often linked process of organisms.

Types of Growth( Importance of Meristem in Growth):

•      The growth may be of two types:

1. Limited growth: The organs like leaves , flowers, and fruits have limited growth. These do not possess meristem
2. Unlimited growth: the organs like stem and roots have unlimited growth. These possess meristem.

Ø  In unicellular plants there is overall growth and not confined to any specific region but in multicellular plants growth is restricted to specific regions having meristematic cells.

Ø  On the basis of their position in the plant body (higher plants) meristem are divided into three main categories:

1. Apical Meristem
2. Intercalary Meristem
3. Lateral Meristem

1. Apical Meristem:

•      Found at root and shoot apex.

•      Activity of these meristems  results increase in length in plants.

2. Intercalary Meristem:

•      Found above the nodes.As a result of activity of these meristems , increase in length takes place.

3. Lateral Meristem:

•      These meristem are made up of cells which divide in radial direction only.

•      They form laterally placed new cells towards the centers and periphery.Cork Cambium and vascular cambium are the examples of lateral meristem. Increase in girth ofshoots and roots take place.

Phases of Growth:

•      In meristems, mitosis occurs which result in two cells resembling the parent cells.

•      Some of these newly formed cell divide again and again forming large number of new cells while in some others the capacity of mitosis is reduced and they starts to elongate and ultimately they get differentiated into vrious type of cells depending upon the function that they have to perform and thus, they become part of the mature organ

•      The phases of growth are as follows:

1. Cell Division               2. Cell Elongation       3. Cell Differentiation

1. Cell Division:

•      It is the first stage of growth. A cell becomes metabolically active at the time of cell division.

•      Its genetic material replicates and the cell divides mitotically into daughter cells.

2. Cell Elongation:

•      It is the second stage of growth, during this state the daughter cells formed as a result of mitosis enlarge.

•      During cell enlargement, new cell wall materials are synthesized in large quantities to cope with the enlargement of the cells.

3. Cell Differentiation:

•      Different tissues are formed as a result of differentiation of newly formed cells.

•      The form of the cells changes and some cells are differentiated into xylem tracheids and trachea and some other into sieve tubes and companion cells.

Course of growth/ Growth Curve (Grand Period of Growth):

•      Growth is slow at first (Lag phase), then gain speed (Log Phase) and eventually slows down (decrease in growth rate).

•      The total time during which this course of growth take place is called as the Grand Period of Growth.

•      In this system, if the rate of growth of a plant or a part of it is measured and is plotted against time , a typical S- Curved is obtained. This curve is known as growth curve.

•      The course of growth can be divided into four parts:

1. Lag Phase
2. Log Phase (Exponential or Linear Phase)
3. Decreasing / Stationary / Steady Phase
4. Death phase or Senescent Phase

1. Lag Phase:

•      It is an initial growth phase. During this phase, little growth occurs. Events that occurs in this phase include germination and vegetative growth.

•      The increased in size or weight is very slow or negligible.

2. Log Phase (Exponential or Linear Phase):

•      It is usually a short phase, and there is rapid growth.

•      The rapid growth rate continues when the plant develop its photosynthetic mechanisms.

•      Therefore , this phase is also called exponential phase or Grand Period of Growth.

•      In all cases of growth, the log phase declines and the rate of growth begins to decrease.

•      The point at which this occurs is known as the inflection point.

•      This phase is called the decreasing phase.

•      The plant activities during this phase include flowering, fruiting, and seed filling.

3. Decreasing / Stationary / Steady Phase:

•      In this phase, the rate of growth declines again after the log phase.

•      In this phase, the flow of food to the vegetative organs is finally ceased.

•      Plant activities in this phase include cell maturation.

4. Death phase or Senescent Phase:

•      After a period of maximum growth, there is a period of negative (senescence) before the death of the plants.

1. Nutrient supply

•      Green plants require several mineral ions and other essential elements for normal growth and development. These nutrients come from the soil for manufacturing of food. Growth ceases when the nutrient supply becomes limiting.

2. Water

•      Water is a medium for all chemical reactions and physiological processes. Water is essential for photosynthesis. Early growth which is due to the turgid conditions of the cells requires plenty of water. Under conditions of water scarcity, plant growth is severely impaired. On the other hand, excess of water in the soil may cause water logging, resulting anaerobic conditions in the roots whose growth is severely affected.

3. Oxygen

•      Oxygen is necessary for cellular respiration in the plants. Food materials are broken down in the process of respiration and energy is released in the form of ATP molecules. This is the utilizable form of energy for the living cells. This energy is used for various activities of the cell and directly takes part in the growth processes.

4. Temperature

•      Plants are influenced by variation of temperature of the soil in which they grow. Most plant species prefer temperature in the range of 20°C to 40°C.

•      Light influences many physiological and growth processes of plants. Plants which can grow well under bright, direct sunlight and grow poorly in shady conditions are called photophilic plants. On the other hand, the plants capable of growing best under low light conditions arc photophobic plants.

•      Light has great morphological effect on the leaves and stems of the plants. Plants growing. in darkness are characterzied by long, succulent weak stems. The leaves become underdeveloped, pale, yellowish, chorophyll deficient.

•      This is called etiolating variations in the intensity, quality and duration of light variously affect seed germination and flowering in different plants. The effect of light on reproductive growth is called Photoperiodism.

6. Growth regulators

•      These are, otherwise, known as hormones. These have pronounced effect on the growth of the plants.